Structures, such as aircraft, civil structures and other large structures, may be built from assemblies, which in turn may be built from subassemblies. In such structures transmitting large loads between one assembly and an adjacent assembly or subassembly is often necessary. For example, one semi-span of an aircraft wing may be attached to a structure on the fuselage. As the wing bends upward due to upward air loads acting upon the wing, compression stress is caused in the upper wing surface and tension loads are created in the lower wing surface. At the root of the wing where the wing attaches to the aircraft fuselage or another semi-span depending on the wing design, transferring the large compression or tension loads from one structure to another may be necessary. Transferring tension loads are more challenging than compression loads for reasons described herein. Structural details or mechanical devices that are often used to transmit these loads are typically referred to as tension clips or tension fittings. Examples of different types of such fittings are illustrated in FIGS. 1-4. The dimensions of the various components of such fittings may vary widely. The different types of fittings may include similar components as described herein. FIG. 1 is a perspective view of an example of a prior art angle clip 100 useable in connecting structures. The angle clip 100 may include an end pad 102. The end pad 102 may include an opening 104 formed therein for receiving a fastener 106, such as a bolt or other type fastener. The fastener 106 may include a shank 109 and a head 110. Opening 104 is sized to prevent the head 110 of the fastener 106 from passing through the opening 104. The end pad 102 may be a plate that carries the fastener load to any adjoining walls by shear and bending forces or loads similar to those illustrated in FIG. 2B. Typically, the end pad 102 is substantially quadrilateral in shape, for example substantially rectangular.
The fastener 106 or bolt may connect the angle clip 100 or other fitting to a mating fitting on an adjacent structure. The angle clip 100 or other fitting may abut a mating fitting on the adjacent structure. An example of a fitting abutting a mating fitting that is attached to an adjacent structure is illustrated in FIG. 14.
All tension clips and fittings described herein have certain features in common related to how they transmit tension loads between two structures: Tension loads are transmitted to a fitting from one structure through the fitting's walls attached (or integral) to that structure and these loads are transmitted to another structure via a tension fastener (or fasteners). FIG. 2B illustrates tension forces acting on the side walls and tension fastener shank 109 of a channel tension clip. Thus, for a pair of mating fittings, highly loaded such that the end pads bend and the side walls between the two adjoining fittings separate from each other, the load path can be described as follows: tension load travels from a structure into the side walls of one fitting to the end pad of that fitting, through the tension bolt into the end pad of the adjoining fitting, and then to the side walls of the that adjoining fitting, and then to the adjoining structure.
The angle clip 100 may include an adjoining wall or side wall 108 that may project substantially perpendicular to the end pad 102 and substantially parallel to an axis of the fastener 106 or bolt. A fitting including three of the four sides of a quadrilateral end pad 102 having adjoining side walls is referred to as a channel fitting. An example of a channel fitting 400 including three adjoining side walls 402, 404 and 406 is illustrated in FIG. 4. The side wall 406 of a channel fitting is also referred to as a back plane. If only two sides that meet in a common corner are joined to the end pad 102, the fitting is termed an angle fitting. An example of an angle fitting 300 including two adjacent joining side walls 302 and 304 is illustrated in FIG. 3. If only one of the sides of the quadrilateral is joined to a side wall, the fitting is termed an angle clip 100 as illustrated in FIG. 1. If two opposite sides of the quadrilateral end pad 102 are each joined to a side wall 202 and 204, the fitting is termed a channel tension clip. An example of a channel tension clip 200 is illustrated in FIG. 2A with the two opposite side walls 202 and 204.
On a weight efficiency basis, channel tension fittings are more efficient than channel fittings, which in turn are more efficient than angle fittings, which in turn, are more efficient than channel or angle clips. While machining cost does influence the design of channel fittings and channel tension clips, minimizing weight of any structural components of an aircraft or structure to be used in outer space is highly desirable. This is because, over the life of the structure, each unit of weight for each part of the vehicle represents a very large amount of fuel with an associated cost. Since the weight savings allows the total vehicle weight to be reduced, there may also be other benefits or advantages, such as for example manufacturing and maintenance costs. The design of the fittings described with reference to FIGS. 1-4 results in a part with a certain weight, depending on such parameters as the axial load, the location of the fastener or bolt with respect to the adjoining walls, and the material properties of the fitting. Accordingly, there is a need for fittings and other components which can reduce weight of a part or assembly without sacrificing structural integrity or incurring a prohibitive manufacturing or maintenance cost.
Additionally, the axial load, as illustrated by arrow 206 in FIG. 2B, from the fastener 106 or bolt is also transferred into the end pad 102 as illustrated in FIG. 2B primarily by the mechanism of the fastener head 110 clamping the end pad 102. From there, the load is transmitted to the side walls 202 and 204 by combined shear and bending forces. In other words, the end pad 102 behaves similar to a beam. Since transferring loads from one point to another point by bending is not as efficient as transferring loads by axial force, there is an inherent inefficiency in using plates in bending to transfer the load.
The use of a bolt forces a certain amount of eccentricity into the connection. Because the bolt has a head which is typically 1.6 times the diameter of the bolt shank, the side walls cannot be any closer than 0.8 times the diameter of the bolt from the axis of the bolt. However, the fittings also need to be constructed with generous fillet radii at the junction of the end pad 102 and side walls 108, 202, 204, 302, 304, 402, 404 and 406 to preclude cracking, further increasing the eccentricity. In addition, unless an internal socket head is used, it is necessary for a socket wrench to fit over the head of the bolt. This minimum eccentricity forces the end pad to be a certain minimum size. For beams, increased length results in increased stresses, which result in inefficiency.
Tension bolts in traditional tension fittings and clips are often sized to have large diameters, in order to increase fitting end pad bending strength. Larger bolt heads increase fitting strength by reducing moments induced in the end pad (specifically by reducing the effective end pad “lever arm” length, the span between the edge of the bolt and the fitting walls). However, this approach to increasing fitting strength results in a weight penalty. The large heavy bolts used frequently end up having greater tension capacity than the fitting itself, which results in structural inefficiency.
The geometry of the fittings and the path of the load through the end pad 102 into the sidewalls require that the locations of high stress due to bending pass through the corners where the side walls 202 and 204 are joined to the end pad 102. This area of the structure has a high stress concentration coefficient for loading as illustrated in FIG. 2B. Thus, even though a generous fillet radius is provided, fittings are susceptible to fatigue cracking at these locations.
Since fittings are often made out of plate or extrusion, there is always a fillet 210, such as fillet 210 in FIG. 2B for which the direction of maximum stress is oriented in the short transverse material direction of the plate 212 as illustrated in FIG. 2B. This material direction is usually the weakest and most brittle direction. Since it is unavoidable to load at least one fillet in this direction, it is necessary to select metallic alloys that are not as brittle. However, the price for this additional ductility is a reduction in ultimate strength. This reduction in allowable stresses results in increased inefficiency of the fitting.
A bolt is comprised of a shank and a head. The shank portion has a threaded portion which accepts the nut that is screwed onto the bolt, and an unthreaded portion. Under axial tension load, the location of maximum stress occurs at the net area under the first thread. Thus, the material of the bolt in the unthreaded area is not loaded to the ultimate capacity of the material because it is limited by the net area under the threads. In addition, the threads introduce a stress concentration due to the notch created by the thread. Thus, a threaded bolt itself has an inherent inefficiency. This inefficiency forces the diameter of the bolt to be larger than it would have been if these effects were not present, which in turn, forces the end pad to be wider than it otherwise would need to be. Thus the inefficiencies in the bolt have a compounding effect on the rest of the fitting. This compounding effect works in the reverse direction also. Increased eccentricities in the joint result in bending forces being applied to the bolt. For the bolt to carry these bending moments, the bolt diameter needs to be increased to sustain them. The increased bolt size therefore results in even greater eccentricity, which compounds itself.
A fitting is machined, forged, or extruded from a single material. Certain parts of the fitting are loaded in tension, while others are loaded in compression or shear. The materials used for current fittings are selected to handle these different loads in different parts of the fitting. This can result in inefficiencies, such as extra weight of the fitting and costs. Accordingly, fittings are needed that take into consideration the different loads carried by different portions of the fittings to be able to more efficiently carry the tension and compression loads and at the same time provide reduced weight and cost.
One challenge in the implementation of conventional channel or angle tension fittings is applying the correct amount of tension preload to the fasteners or bolts joining the end pads of two fittings together. At least three aspects of particular concern are: 1) the consequences of overloading the bolt may be significant in terms of possible breaking of the fitting and cost of repair or replacing; 2) if the preload is too little, the fitting may allow the two structures or components being fastened to open up or separate; and 3) determining the actual preload in the bolt or fastener may be a process that contains a significant amount of uncertainty when using the most common methods for bolt pre-tensioning.
Examples of methods for controlling bolt tension or strain during installation may include using a torque wrench, turning a nut or bolt plus or minus a preset percentage when tightening, using direct tension washers, applying strain gages, using special bolts with built-in extension measurement, and using hydraulics to heat or pretension bolts. Using a torque wrench and turning a nut or bolt a preset percentage are low cost techniques but inaccurate. Direct tension washers are more accurate, but more expensive, and their use may be limited to certain applications or industries. Strain gages, special bolts with built-in extension measurement and hydraulic or heat to pretension bolts have better accuracy but can be expensive and difficult to implement.
In conventional angle and channel tension fittings, such as those described with reference to FIGS. 1-4 above, the flat end pads 102 may be compressed and plastically deform if the tension bolt or fastener 106 is overloaded. In traditional angle or channel tension fittings, there is no accurate and inexpensive way to directly measure the state of stress, strain, or displacement of the fastener and/or fitting, because there is no easy way to obtain access to the shank 109 of the fastener 106.